NMR study of the complexation of D-galactonic acid with tungsten (VI) and molybdenum (VI)

By using multinuclear (1D and 2D) magnetic resonance spectroscopy, D-galactonic acid was found to form four complexes with tungsten(VI), in aqueous solution, depending on pH and concentration conditions: two isomeric 1:2 (metal:ligand) complexes involving the carboxylate and the adjacent OH group, and, at high pH, two 2:1 complexes involving the four OH groups of the central part of the carbon chain. Homologous species were found with molybdenum(VI), in similar conditions, but four additional complexes were detected: two 4:2 isomers, at low pH, involving the carboxylate and all the hydroxyl groups, and two 2:1 complexes at intermediate pH. These complexes are not present in the case of W(VI) because of competition of polymerization equilibria of tungstate at low pH. Detailed structures are proposed for most of the complexes and formation constants are estimated for the W(VI) complexes. © 1997 Elsevier Science Ltd.